How long aftershocks

Faults can extend deep into the earth and may or may not extend up to the earth's surface. Some faults have not shown these signs and we will not know they are there until they produce a large earthquake. Several damaging earthquakes in California have occurred on faults that were previously unknown.

Surface features that have been broken and offset by the movement of faults are used to determine how fast the faults move and thus how often earthquakes are likely to occur. For example, a streambed that crosses the San Andreas fault near Los Angeles is now offset 83 meters 91 yards from its original course. The sediments in the abandoned streambed are about 2, years old.

If we assume movement on the San Andreas has cut off that streambed within the last 2, years, then the average slip rate on the fault is 33 millimeters 1.

This does not mean the fault slips 33 millimeters each year. Rather, it stores up 33 millimeters of slip each year to be released in infrequent earthquakes. The last earthquake offset the streambed another 5 meters 16 feet. If we assume that all earthquakes have 5 meters millimeters of slip, we will have earthquakes on average every years: millimeters divided by 33 millimeters per year equals years.

This does not mean the earthquakes will be exactly years apart. While the San Andreas fault has averaged years between events, earthquakes have occurred as few as 45 years and as many as years apart. Step 7: When safe, follow your disaster plan. Earthquake Basics Epicenter, hypocenter, aftershock, foreshock, fault, fault plane, seismograph, P-waves, magnitude, intensity, peak acceleration, amplification Earthquakes and Faults What is an earthquake?

When a relatively large earthquake occurs, a series of smaller earthquakes will follow continuously around the epicenter of the first earthquake. A large earthquake whose epicenter is shallow accompanies aftershocks in most cases. Unlike mainshock-aftershock type, it does not have particular large earthquakes; however its seismic activity lasts for a long period of time, varying its level of the activities.

It is the area where aftershocks occur. Till about the first 24 hours after the occurrence of a main-shock, its aftershock area almost accords with the hypocentral area, an area destroyed by the main-shock. But for faults where the sides scraped past each other at just a few millimeters per year, aftershocks lasted about years, the researchers reported. The longest series of aftershocks, some which have lasted several centuries, were triggered by quakes that occurred in continental interiors along slow-moving faults.

Large earthquakes are often followed by aftershocks, the result of changes in the surrounding crust brought about by the initial shock. Aftershocks are most common immediately after the main quake. As time passes and the fault recovers, they become increasingly rare.

This pattern of decay in seismic activity is described by Omori's Law but Stein and Liu found that the pace of the decay is a matter of location. At the boundaries between tectonic plates, any changes wreaked by a big quake are completely overwhelmed by the movements of the plates themselves. At around a centimetre per year, they are regular geological Ferraris.

They soon "reload" the fault, dampen the aftershocks, and return the status quo within 10 years. In the middle of continents, faults move at less than a millimetre every year.

In this slow lane, things can take a century or more to return to normal after a big quake, and aftershocks stick around for that duration. Again, New Madrid proves the principle - a cluster of large earthquakes hit the area in the past thousand years, but the crust shows no sign of recent deformation according to two decades of GPS measurements.

It seems that recent activity really is the legacy of centuries-old quakes, a threat that has since shut down. They happen on the faults we think caused the big earthquakes in and , and they've been getting smaller with time. To test this idea, Stein and Liu used results from lab experiments on how faults in rocks work to predict that aftershocks would extend much longer on slower moving faults.

They then looked at data from faults around the world and found the expected pattern. For example, aftershocks continue today from the magnitude 7. This might be of some comfort to residents near the epicenter of the Hebgen Lake Quake. Then again, it might not. It's rather hard to feel comforted by the fact that the fault moves slower than the San Andreas, and therefore shall have aftershocks longer, when the last big quake took down a mountainside, ripped open roads, created a new lake, and left fault scarps all over the danged place, right?

This limits the earthquake size, because there is not much left to rupture, he said. Van der Elst said larger aftershocks start at the edge of the earthquake tear, where the quakes can break new ground on unbroken fault areas. Aftershocks can also strike off the fault entirely, as the surrounding rock adjusts to its new position. While the results make intuitive sense, demonstrating that nature follows these statistical patterns is the first step toward including aftershock location in official forecasts.

A forecast model that includes location statistics would likely be useful in places such as California, where public agencies must rapidly respond to the threat of aftershocks, said Ned Field, a USGS research scientist in Golden, Colorado, who was not involved in the study.

To make matters more complicated, there are some special cases involving aftershocks. Occasionally an aftershock is larger than the initial earthquake. In this case, geologists rename the first earthquake, calling it a foreshock, and the aftershock becomes the primary earthquake.

Also, there are usually other faults nearby that have built up strain over the years. A nearby earthquake may push these faults over the edge. These events are not considered aftershocks, however, because the added stress from the earthquake was just the tipping point that triggered the fault to release its pent-up energy, resulting in a new earthquake.

Follow Becky Oskin beckyoskin. Originally published on Live Science. Copyright LiveScience , a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Already a subscriber? Monitor journalism changes lives because we open that too-small box that most people think they live in. We believe news can and should expand a sense of identity and possibility beyond narrow conventional expectations.

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